In a radio communication method such as CDMA, for example, it is important to realize a high-speed information rate. One known signal transmission method uses MIMO (Multiple-Input Multiple-Output) channels with plural transmission/reception antennas in order to achieve such a rate. In the MIMO transmission method, both a transmission side and a reception side have N disposed antennas and plural different signals are efficiently transmitted at the same time using the same frequency band through a network with N port inputs and N port outputs connected via a radio circuit. In other words, the method is intended to enlarge the capacity of transmission by increasing the numbers of transmission antennas and reception antennas so as to use space in a multiple manner.
In the MIMO multiplexing method as shown in FIG. 1, when plural different transmission symbols are transmitted at the same time with each of the N transmission antennas 124 using the same frequency and the same spread code, these transmission symbols are synthesized in space. This can be interpreted that the transmission symbols are multiple-valued in space in a certain sense, so that the information rate can be increased to several times that of the transmission antennas.
In this MIMO multiplexing method, known techniques for realizing a high-reliability transmission employ error-correcting encoding and interleaving. For example, conventional techniques are disclosed in “Takumi ITO, Xiaodong WANG, Yoshikazu KIMURA, Mohammad MADIHIAN, and Akihisa USHIROKAWA “MF and MMSE Combined Iterative Soft Interference Canceller for MIMO/OFDM Systems” The Technical Report of the Institute of Electronics, Information and Communication Engineers of Japan, RCS2002-295, pp. 117-124, March, 2003″ (Non-patent Document 1).
FIG. 2 shows an example of a conventional MIMO transmission system. A transmitter 210 comprises an error-correcting encoder 214, an interleaver 218, a serial-to-parallel converter 212, and N antennas 224. A receiver 240 comprises N antennas 254, a signal separator 252, a parallel-to-serial converter 242, a deinterleaver 248, and an error-correcting decoder 244. Interleaving is a process for switching the order of encoded bit data series prior to modulation and performing the reverse operation after demodulation. The interleaving is used to separate and relocate burst errors exceeding several code words (block codes) or a constraint length (trellis codes) in order to accurately decode exact random errors with a high probability in accordance with designed codes.
In the transmitter 210, transmission data 211 are encoded with error-correcting codes, interleaved, and then resultant serial data is serial-to-parallel converted, thereby gaining N parallel data sets. Each parallel data set is transmitted using the corresponding transmission antenna 224.
Then, each of the antennas 254 of the receiver 240 receives signals transmitted from the transmitter 210. The received signals are separated into N parallel signals using the signal separator 252 of the receiver 240. The N parallel signals after the signal separation are parallel-to-serial converted, deinterleaved, and the error-correcting codes are decoded.
In this example of conventional techniques, information before the serial-to-parallel conversion is encoded with error-correcting codes and interleaved, so that improved effects on the characteristics of an error rate are expected using a space diversity effect.
Non-patent Document 1: “MF and MMSE Combined Iterative Soft Interference Canceller for MIMO/OFDM Systems” by Takumi ITO, Xiaodong WANG, Yoshikazu KIMURA, Mohammad MADIHIAN, and Akihisa USHIROKAWA, The Technical Report of the Institute of Electronics, Information and Communication Engineers of Japan, RCS2002-295, pp. 117-124, March, 2003
For example, in a CDMA mobile communication system such as WCDMA, CDMA 2000, and the like, it is required that an ultra high-speed information rate be realized. And regarding that requirement, it is possible to increase the information rate by applying the MIMO multiplexing method as mentioned above in which parallel transmission of information is performed using plural transmission antennas. However, when a conventional structural method as shown in FIG. 2 is used, very high-speed processing is required in the error-correcting encoder 214 and the interleaver 218 of the transmitter 210, and in the deinterleaver 248 and the error-correcting decoder 244 of the receiver 240. Also, the size of the interleaver and the deinterleaver must be enlarged. For example, when information is transmitted on the assumption that the number of the transmission antennas is N=4 and the information is transmitted at 250 Mbps in each antenna, it is required that the error-correcting encoder 214 and the interleaver 218 of the transmitter 210, and the deinterleaver 248 and the error-correcting decoder 244 of the receiver 240 process data at 250×4=1000 Mbps=1 Gbps. This high-speed processing imposes a heavy work load in terms of implementation and thus poses a problem.